Acid rock drainage (“ARD”) is generated by either natural events or a combination of human activity and natural events and is widely known as an important environmental problem. ARD is the product formed by the oxidation of such commonly found iron-sulfide minerals as pyrite (FeS2) and pyrrhotite (FeS). Human activities, such as mining and other rock excavation activities, or disposal of waste, may promote the generation of ARD by increasing the quantity of sulfides exposed to atmospheric elements, thereby increasing the sulfide oxidation process.
While the general characteristics of ARD may vary, ARD is typically acidic with elevated sulfate and dissolved metal concentrations. The consequence of ARD is that any area that is covered by sulfuric materials and wastes, including waste rock, tailings management areas, sulfide concentrate dumps and excavated mine openings and heap leach piles can adversely affect downstream ecology by decreasing the quality of receiving ground and surface waters. This is because water that infiltrates sulfidic materials in which sulfide minerals have oxidized may become acidic and contaminated with elevated concentrations of sulfate and dissolved metals including iron, copper, zinc, and others. The contaminated drainage water is commonly referred to as ARD, acid mine drainage (AMD), and acidic drainage (AD).
The sulfide oxidation process can be generally represented by oxidation of pyrite (FeS2), a common iron sulfide mineral. The following equations describe the oxidation process for pyrite. Equation 1 shows that pyrite oxidation in the presence of sufficient quantities of oxygen and water produces sulfate, ferrous iron, and acidity (H+). As shown in equation 2, the ferrous iron (a reaction product in Equation 1) may oxidize to form ferric iron. Equation 2 can be catalyzed by bacteria (such as Thiobacillus ferrooxidans) under low pH conditions. As shown by equation 3, the ferric iron may then hydrolyze to form ferric hydroxide and acidity.FeS2+3.5O2+H2O→2SO42−+Fe2++2H+  (1)14Fe2++3.5O2+14H+→14Fe3++7H2O  (2)Fe3++3H2O→Fe(OH)3+3H+  (3)Alternatively, depending upon chemical conditions, ferric iron may be consumed to oxidize more pyrite and produce additional acidity as shown by equation 4.14Fe3++FeS2+8H2O→15Fe2++2SO42−+16H+  (4)Equation 5 represents acid generation where iron is precipitated as Fe(OH)3. An overall reaction for ferric iron that reacts to oxidize pyrite is shown by equation 6.FeS2+15/4O2+7/2H2O→Fe(OH)3+2SO42−+4H+  (5)FeS2+15/8O2+13/2Fe3++17/4 H2O→15/2Fe2++2SO42++17/2  (6)As indicated by equation nos. 1 to 6, the sulfide oxidation process is initiated in the presence of oxygen.
The rate of acid generation, once initiated, is determined primarily by factors that include the solution pH, the oxygen content of the gas phase, the oxygen concentration in the water phase, the degree of moisture saturation in interstitial pore spaces, ferric iron activity, and the exposed sulfide surface area.
The products of sulfide oxidation may be promptly flushed from the sulfide oxidation sites by water or, may accumulate until flushed by infiltrating water. When acid products are flushed away from a sulfide mineral oxidation site, they may encounter acid consuming (e.g. acid buffering) mineralization that may neutralize all or a portion of the free and metal acidities.
Another closely related problem is that of “metal leaching” where the pH of the acid drainage solutions are neutralized, but still contain elevated dissolved metal concentrations. At such sites, where elevated dissolved metal concentrations result from the buffered acid drainage, there is also adverse impact on the receiving environment. For example, elevated concentrations of dissolved zinc or copper can adversely affect the receiving aquatic environment due to their toxicity in sufficient concentrations.
Physical factors also affect the rate of acid generation. For sulfidic waste, physical factors including rock permeability are important. Sulfidic waste with high permeability and uncovered, unconsolidated surfaces may have high oxygen ingress, which in turn may contribute to increased oxidation rates. These higher rates along with higher internal temperatures due to exothermic oxidation, generally help drive convection and the ingress of oxygen from the atmosphere to sulfide minerals contained within a mass of sulfidic wastes.
Studies have indicated that the prevention/reduction of sulfide oxidation at its source(s) can be accomplished by inhibiting the input of oxygen, which is the principle ingredient for the initiation of the sulfide oxidation and the ARD generation process. Further, it should be understood that even with available oxygen and no water infiltration, the moisture content of a mass of sulfidic waste (for example, a mine waste rock dump) may be sufficient to allow sulfide oxidation and in-situ acidification to occur.
Prior art methods to alleviate the problems associated with ARD include the use of engineered dry covers, the blending of alkaline materials to assist in neutralizing acidity in situ, and underwater disposal and in situ flooding. For example, U.S. Pat. No. 6,004,069 issued Dec. 21, 1999(Sudbury) discloses a method of constructing a composite dry cover used in the isolation and encapsulation of sulfide-bearing wastes, including rock dumps. Dry covers are generally designed to attenuate the influx of atmospheric oxygen and/or water into the underlying sulphidic wastes, consequently reducing/preventing and controlling acid generation in sulphidic wastes capped by dry covers. While dry covers may be custom designed to incorporate several layers and appropriately graded slopes, the uncertainty of the long-term performance of dry covers due to the environment (i.e. settlement, frost heaving, cracking, plant root evasion, erosion, geotextile degradation, etc. . . ), raises concerns as to their overall effectiveness in ARD prevention and control. In addition, such prior art dry covers, depending on the raw material available in the vicinity of the mass of sulfidic waste, are generally expensive to construct and to maintain.
Also, U.S. patent application Ser. No. 2001/0032725, filed Mar. 6, 2001(Harrington) discloses a process to treat acid rock dumps by three general methods (physical, chemical, and biological), and thereby reduce ARD. Harrington does not teach control of the internal atmosphere of a mass of sulfidic waste by which ARD reduction is achieved by monitoring and reacting to changing conditions. Harrington does not substantially provide the flexibility to address progressive cover degradation or changes in barometric pressure. Harrington further does not sufficiently address the impact of climactic conditions on ARD reduction.
While a significant amount of research and a number of technologies have been targeted at the prevention and control of ARD generation, prior art solutions generally focused on inhibiting the input of atmospheric oxygen. What is required is a solution whereby a substantially chemically inert internal atmosphere within the mass of sulfidic waste is provided and maintained in changing conditions. There is a further need for a method and apparatus for reducing ARD generation in relation to a mass of sulfidic waste that is easy and relatively inexpensive to apply.